In zig-zag sewing machines, the sewing needle, in addition to its usual vertical reciprocability, is transversely shiftable for implementation of zig-zag stitching effects, and also still more complicated stitch patterns. Also, the feed dog of the machine's cloth feeder is generally designed to be able to implement both forwards and reverse feed of sewn material. Complex or otherwise problematic stitch patterns are implemented by transversely shifting the machine's needle to predetermined positions, and by operating the feed dog to implement forwards or backwards cloth feed by predetermined increments, in preparation for each needle penetration to be performed during the ongoing course of sewing a complex or problematic stitch pattern. Even assuming that the amount by which the needle is transversely shifted and the sewn material fed forwards or backwards by the feed dog in preparation for each needle penetration can both be absolutely controled, implementation of complex stitch patterns requires good judgement in the dimensional matching of this combination of needle-shift and cloth-feed amounts for each successive needle penetration and, more difficult, of all such combinations of amounts for all the successive needle penetrations of the whole stitch pattern to be implemented. Typically, this is not done, for example, using a pencil and graph paper in the planning stage, but instead is worked out on cloth workpieces during actual sewing of such workpieces, i.e., proceeding on a trial-and-error basis.
Actually, there is no other feasible approach to the planning or setting-up of a complex stitch pattern; for example, plots of needle-penetration locations on graph paper are in general quite inadequate for such preliminary work. The reasons for this situation are, of course, familiar to persons skilled in the art. In principle, the transversely displaced position to which the machine's needle is to be shifted prior to the next constituent needle penetration involved in a stitch pattern can, on most zig-zag sewing machines, be predetermined with a very high degree of definiteness; and to this extent, plotting of the successive needle-penetration points of a fancy stitch pattern on graph paper or the like would, in principle, be meaningful. However, where the geometry or configuration of the stitch pattern to be implemented depends upon the feeding of the sewn cloth by non-constant increments for the successive needle penetrations of the pattern or, even worse, where cloth-feed direction reversals are necessary for implementing the pattern, the situation becomes very problematic.
It is a notorious problem in the art that the amounts by which a sewing machine's cloth-feeding feed dog incrementally advances the material being sewn are very difficult to predetermine. To implement successive increments of cloth feed of successive different amounts for the successive needle penetrations, it is of course typical to correspondingly adjust the machine's cloth-feed unit for each successive stitch. However, the simple fact is that, although the cloth-feed unit is nominally adjusted to implement the cloth-feed increments required, the cloth-feed increments physically obtained simply do not match those which have been thusly set on the machine's feeding mechanism; this is due to problems of uncontrollable cloth shifting during cloth feed, and in general due to problems of inherently indefinite operation, or indefinite adjustability, of sewing-machine cloth feeders. The situation is greatly worsened when feed-direction reversals are necessitated. Even if the amount of the cloth-feed increment is kept constant, when one cloth-feed increment is performed, followed by a needle penetration, followed by a nominally identical cloth-feed increment in the opposite feeding direction, the opposite-direction feed increment which actually results will, most often, simply not be of the same magnitude as the previous one. This is a very well known problem in the art.
As a result, considerable amounts of trial-and-error work, performed on actual workpieces, is typically necessary, in the course of setting-up for the sewing of a particular stitch pattern.
By way of example, attention is directed to the stitch patterns depicted in FIGS. 2a-2e.
FIG. 2a depicts a conventional buttonhole stitch pattern. As is well known, an acceptable buttonhole stitch pattern involves proper establishment of the cloth-feed increment E used for the left-side line-tack stitches, proper establishment of the cloth-feed increment F used for the right-side line-tack stitches, proper establishment of the zig-zag amplitude A employed for the upper bar-tack stitches and for the lower bar-tack stitches B, and proper establishment of the width D of the central unstitched zone on the sewn material where the buttonhole slit is to be formed. As is extremely well known, even if the amount of the cloth-feed increment used for one of the two sets of line-tack stitches is again utilized during the opposite-direction feeding of cloth performed during the sewing of the other set of line-tack stitches, the amount of the reverse-direction feed increment actually achieved will often be different, leading to differing numbers of constituent stitches for the left and for the right rows of line-tack stitches.
FIGS. 2b-2e depict four different stitch patterns, complex ones requiring frequent reversals of the cloth-feed direction. In each of these four Figures, at the bottom of the Figure the stitch pattern is shown sewn correctly, and at the top of the Figure there is shown the sort of difficulties which arise during the course of trying to achieve the respective stitch pattern. In FIG. 2b, because of the problematic controllability of the amount of the cloth-feed increment encountered upon feed-direction reversals, a gap, of length C measured in the cloth-feed direction, develops within the stitch pattern. Similar gaps, their lengths likewise designated by C, are shown developed in the stitch patterns depicted in FIGS. 2c and 2d. In FIG. 2e, which depicts a so-called "leaf" pattern, the pattern as it should appear is depicted at the lower part of the Figure. The problem arising is depicted at the upper part of the Figure and is here constituted, not by a gap of length C, but instead by a backwards-going overlap of length C; i.e., the lower tips of successive "leaves" of the so-called "leaf" pattern are improperly extending into the bodies of their neighboring "leaves". When a stitch pattern as complicated as that of FIG. 2e is involved, i.e., involving a cloth-feed increment whose magnitude changes for almost each successive one of the constituent needle penetrations, the trial-and-error work involved in getting this stitch pattern to "go right" is very considerable indeed.